Compounds containing high levels of aromatically-bound bromine have been used extensively for many years. An excellent example is decabromodiphenyl oxide. This compound is characterized by efficiency as a flame retardant additive anid good thermal stability.
More recently, oligomeric flame retardant additives have been used in commercial polymers. These compounds also contain aromatically-bound bromine. They melt at lower temperatures than the type of compound cited above and blend readily with many thermoplastic resins. A benefit of the oligomeric flame retardants is the improved melt flow of thermoplastic resins containing them.
Another advantage of these higher molecular weight flame retardants is their resistance to migration when incorporated into a polymer matrix. Migration is observed as a powdery deposit on the surface of the polymer or on the surface of a mold.
Further, it has been shown that higher molecular weight compounds, such as oligomers, tend to be less soluble, and therefore less toxic, than smaller, lower molecular weight compounds.
Each type of oligomeric flame retardant introduced thus far has inherent disadvantages. For example, uncapped brominated epoxy oligomers exhibit black specks that are formed during compounding and decreased melt flow due to chemical interaction. Even when capped, the epoxy oligomers still possess potentially reactive hydroxyl groups.
Carbonate oligomers also need to be capped. They tend to be unstable during compounding. Moreover, acrylate oligomers are prone to depolymerization under heat and shear. Furthermore, their ester linkages can also hydrolyze over time.
The lower bromine content of current oligomeric flame retardants reduces their efficiency. The higher loading requirements to achieve the desired flammablility test performance often result in decreased physical properties.
Moreover, concern exists that some of these compounds (such as brominated diphenyl oxides) may be hazardous to human health. It has been suggested that compounds having oxygen atoms attached to aromatic rings may be altered under certain thermal and atmospheric conditions to yield toxic dioxins and dibenzofurans.
In designing new chemical structures, it may be deemed advantageous to avoid oxygenated aromatic species. A more recently introduced flame retardant, decabromodiphenyl ethane (U.S. Pat. No. 5,302,768), has a high bromine content and no oxygen on the aromatic rings. However, its high melt range and tendency to migrate, particularly in olefinic resins, are drawbacks.
There is still a need for a flame retardant additive that contains no aromatic oxygen, is thermally stable, exhibits lack of bloom, good melt flow of the oligomers and has a high bromine content.